Covalent anthocyanin-flavone dimer from leaves of Oxalis triangularis |
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Authors: | Fossen Torgils Rayyan Saleh Holmberg Maya H Nimtz Manfred Andersen Oyvind M |
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Institution: | Department of Chemistry, University of Bergen, Allégt. 41, N-5007 Bergen, Norway. Torgils.Fossen@kj.uib.no |
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Abstract: | The anthocyanin-flavone C-glycoside, (malvidin 3-O-(6(II)-O-alpha-rhamnopyranosyl(AIV)-beta-glucopyranoside(AII))-5-O-beta-glucopyranoside(AIII)) (apigenin 6-C-(2(II)-O-beta-glucopyranosyl(FIII)-beta-glucopyranoside(FII))) malonate(AV) (A(IV)-4-->A(V)-1, F(III)-6-->A(V)-3) (1), has been isolated from leaves of Oxalis triangularis A. St.-Hil. In the 1D (1)H NMR spectrum of 1 dissolved in CD(3)OD-CF(3)CO(2)D (95:5), MTFA, recorded 45 min after sample preparation, this covalently linked dimer occurred mainly as flavylium cation (38%) and two equilibrium forms assigned to be quinonoidal bases (54%), whereas only minor amounts of the hemiacetal forms were present. After five days storage at 300 K, the hemiacetals (39%) and flavylium cation (38%) constituted the main forms of 1. More simple anthocyanins are normally considered to be on the flavylium cation form in acidified deuterated methanol. The cross-peaks observed in NOESY NMR spectra of 1 indicated the presence of vertical 'pi-pi' stacking between the B-ring of the flavone unit and the A-ring of each of the two forms assigned to be quinonoidal bases. It was not possible to discriminate between inter- or intramolecular association mechanisms. The equilibria between the various forms of 1 were studied by two-dimensional NOESY and ROESY NMR spectroscopy. 2D HSQC-TOCSY NMR spectroscopy was among the methods used for characterization of the various forms. |
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Keywords: | Oxalis triangularis Leaves Anthocyanin-flavone C-glycoside dimer Malvidin 3-rutinoside-5-glucoside (malonate) apigenin 6-C-sophoroside Colour Equilibrium forms HSQC-TOCSY 13C NMR |
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